Remark 2.21 Definitions (1) – (2) on the unipotent stabilisers appear in [1]}, [2]}, whereas (3) is an additional condition that we use in this paper to determine the stable locus explicitly. Condition (4) is a weakened version of (2): when (2) fails, we will perform a blow-up sequence to arrange for (2) to hold, and (4) ensures that we never blow up all of \(Z_{\min }\)  and so we can easily determine the minimal weight space in the blow-up and obtain an explicit expression for the stable set in this case.

